Multi-ply disintegratable absorbent sheet, associated roll and associated manufacturing process

ABSTRACT

The invention relates to a substantially rectangular multi-ply disintegratable absorbent sheet based on tissue which comprises at least three plies, and which has a coefficient
 
 K   NOVE   =R   SM   ×R   ST   ×A×G×Exp (12×( E+E   p ))×(1/ Exp  ( SP ))&gt;75 000
 
where
         R ST =strength in the cross direction of the sheet, in N/m   R SM =strength in the machine direction of the sheet, in N/m   G=basis weight of the sheet in kg/m 2      E=thickness of the sheet in mm   Sp=flexibility of the sheet in N   A=absorption of the sheet in kg/m 2      E P =mean thickness of one ply of the sheet in mm, E P  being greater than 0.115 mm.       

     The invention is also aimed at the process for manufacturing such a sheet and at rolls consisting of said sheets.

This application is a National Stage entry of International ApplicationNo. PCT/FR2007/000625, filed Apr. 13, 2007, which claims priority toEuropean Patent Application No. 06290613.6, filed on Apr. 14, 2006, thepriorities of which are hereby claimed and their disclosuresincorporated by reference in their entirety.

The present invention relates to the field of sheets of absorbent papermade of fibrous material such as tissue, comprising at least threeplies, preferably five or more.

The preferred application of the invention is that of paper for sanitaryor domestic or even cosmetic use, such as bathroom tissue, but alsokitchen towel, paper handkerchiefs or alternatively the “formats” thatcan be used for cleansing the skin and/or removing makeup.

In general, this type of product needs to display a certain number ofcharacteristics such as softness, flexibility, dry and/or wet strength,absorption, thickness, and also disintegratability, etc.

One or more others of these parameters will take precedence depending onthe anticipated application, and known products thus display either goodsoftness or excellent absorption, or perfect strength, etc.

Furthermore, the visual appearance is also a parameter that needs to betaken into consideration: the esthetic look of known absorbent productscan be achieved by embossing the sheet or alternatively by printingpatterns using ink, these patterns varying in their visibility andharmonious, or alternatively by combining both techniques.

Where toilet paper is concerned, the objective will be to obtain goodstrength, a certain thickness and great softness. The strength andsoftness may seem difficult to reconcile but by combining certain fibersand or certain basis weights with a treatment such as calendering, goodresults can be obtained.

However, the known products do not exhibit all the desiredcharacteristics listed above; a choice is therefore generally made,leading to a product that displays one or two of the above listedtechnical characteristics.

It is an object of the present invention simultaneously to achieve goodperformance across all of the characteristics.

One approach which may appear simple and obvious is to associate a greatnumber of plies to form a very thick absorbent sheet. This approach hashitherto remained a theoretical one because technical problems occurwhen attempts to associate four or more plies are made: either it isnecessary to provide several adhesive application stations, or it isnecessary to provide mechanical association means that entail a strongpressure on each of the plies of which the sheet is formed.

The adhesive application stations each lead to problems of soiling,maintenance, floor space on the manufacturing line, etc., leading toadditional costs and/or complications in the manufacture.

On the other hand, excessively high pressures lead to premature wear ofthe manufacturing rollers or cylinders, crush the embossed patternsand/or lead to vibrations which prove to be detrimental not only to thecylinders themselves but which also have a negative effect on thequality and reliability of the sheets produced.

Patent application WO 97/20107 describes a sheet of multi-ply absorbentpaper in which the plies are associated in the converting phase using amethod known as “dry marking” which consists in deforming the sheetthrough its thickness under stress by passing it between an engravedroller with a rigid external surface and a smooth roller, the externalsurface of which is less hard than that of the engraved roller.

This then yields a multi-ply product where the plies are associatedpurely mechanically, and which is marked in certain regions.

Furthermore, sheets of absorbent paper which are very thick and/or usedsimultaneously in excessive quantity may lead to blocked pipes.

However, there is a need, particularly although not exclusively as faras bathroom tissue is concerned, for a paper that is at once thicker,strong, soft, flexible, absorbent and disintegrates readily.

Indeed, with a paper that displays all of these characteristics, theuser can use just one format (or sheet) at a time, that is to say foreach wipe, hence making not insignificant savings when it is borne inmind that with known bathroom tissue, several (four or five) sheets arecommonly employed by the consumers on each use.

Thus, by using fewer sheets each time, the risk of blockages in thepipes is greatly reduced.

The present invention proposes a solution which is unobvious, simple,reliable and can be adapted to suit several types of production.

Bathroom tissue is a preferred application of the invention but theinvention can also be used for other products such as formats forcleaning, removing makeup or other skincare applications, and as kitchentowel, paper handkerchiefs, etc.

A subject of the present invention is a substantially rectangularmulti-ply disintegrable absorbent sheet based on tissue and comprisingat least three plies.

Furthermore, the sheet has a coefficient K_(NOVE)=R_(SM)×R_(ST)×A×G×Exp(12×(E+E_(P)))×(1/Exp (SP))>75 000 where

-   -   R_(ST)=strength in the cross direction of the sheet, in N/m    -   R_(SM)=strength in the machine direction of the sheet, in N/m    -   G=basis weight of the sheet in kg/m²    -   E=thickness of the sheet in mm    -   Sp=flexibility of the sheet in N    -   A=absorption of the sheet in kg/m²    -   E_(P)=mean thickness of one ply of the sheet in mm, E_(P) being        greater than 0.115 mm.

According to another feature, the absorbent sheet also has a coefficientK_(D)=K_(NOVE)×exp (D)>100 000,

D being a softness value ranging between −3 and +3.0.

Notable softness is thus obtained according to the invention for a sheetwhich is also both thick and flexible.

According to the invention, the absorbent sheet has a tensile strengthin the machine direction greater than about 400 N/m, preferably rangingbetween 400 and 1000 N/m, preferentially higher than 700 N/m.

Preferentially, the sheet has a tensile strength in the cross directionranging between about 200 N/m and about 500 N/m, preferentially higherthan 300 N/m.

Thus, such an absorbent sheet simultaneously displays properties ofsoftness, thickness and exceptional tensile strength both in the machineand in the cross directions.

Particularly, the sheet may comprise between 3 and 12 plies,preferentially between 3 and 9 plies; it being possible for each plyitself to comprise a group of plies.

Advantageously, the sheet may have a thickness greater than about 0.75mm and less than about 1.5 mm.

This thickness encourages the user to use a small number of sheets,preferably just one, each time.

Particularly, the basis weight of the sheets according to the inventionis greater than or equal to about 70 g/m² and preferentially less than150 g/m².

The sheets may have a length of about 140 mm and a width of about 110mm, particularly for uses as bathroom tissue.

A format such as this is somewhat larger than the format of knownsheets, hence yet again discouraging the use of several sheets at atime.

Furthermore, in the context of its use as bathroom tissue, the rollsaccording to the invention may comprise between 60 and 120 sheets, whichis markedly fewer than conventional rolls of toilet paper. Of course,given the intrinsic characteristics of each sheet, this does not causethe user to bear an additional cost because one of the significantadvantages of the invention lies in the use of just one sheet instead ofthree, four or even more each time.

This then yields a significant financial saving, which is very muchliked and appreciated by consumers.

Furthermore, in spite of the great thickness of the sheets, rolls can beformed without any problems. This is made possible by the intrinsiccharacteristics of the sheets according to the invention whichsimultaneously display remarkable elasticity and remarkable flexibilitywhich are both entirely compatible with rolling, even at the center ofthe roll where the radii of curvature are the smallest.

Furthermore, another subject of the invention is a process formanufacturing disintegratable absorbent sheets which consists inembossing at least three of the plies that form said sheet separatelyand differently, in chemically associating said sheets using a singleadhesive-coating unit then in mechanically associating the collection ofplies using a press cylinder.

Without departing from the scope of the invention, the manufacturingprocess may consist, in the wet phase, in throwing a suspension ofpapermaking fibers onto a fabric in order to form a web, in transferringthis web to the surface of a porous drying cylinder, in making jets ofair from said cylinder to pass through the web, then in transferring thedry web thus formed onto a creping cylinder. Several plies are thusformed independently of one another and then, later, in the so-calleddry phase, said plies are associated with one another by mechanicaldeformation.

According to a preferred embodiment of the invention, at least one ofthe external surfaces of the sheet is obtained using this process.

With these processes, the minimum specific pressure for associating theplies is of the order of 3 kg/mm².

The invention is also aimed at a multi-ply disintegratable absorbentsheet comprising plies obtained by one of the processes described aboveand/or plies obtained by the other of the aforementioned processes.

Other characteristics, advantages, details of the invention will becomebetter apparent from reading the description which will follow, given byway of entirely nonlimiting illustration with reference to the attacheddrawings in which:

FIG. 1 is a schematic section through the thickness of a sheet producedaccording to one of the embodiments of the invention;

FIG. 2 is a diagram illustrating an installation for the manufacture ofthe sheets according to one of the embodiments of the invention;

FIG. 3 is a simplified perspective view of a roll obtained according tothe invention;

FIG. 4 is a schematic perspective of the test mechanism for measurementof Sp flexibility illustrating the cage, cone and rod along with theconfiguration for a standard test sample;

FIG. 4A is a sectional view illustrating the profile of flaring hole104;

FIG. 5 is a schematic perspective of the test mechanism in operationmeasuring the Sp flexibility of a standard test sample as the cone pullsit through the hole in the cage.

According to FIG. 1, a multi-ply sheet according to the invention maydisplay a structure having at least three plies or groups of plies 1, 2,3.

According to one of the embodiments of the invention, each ply or groupof plies is embossed independently (as will be explained in greaterdetail in relation to FIG. 2) and differently.

By way of illustration, the first ply (or group of plies) 1 may be anouter ply, embossed with a so-called decorative pattern; the second ply2 (or group of plies) is positioned on the inside of the sheet andcomprises coarse microembossing; the expression “coarse microembossing”must be understood for example to mean microembossing comprisingapproximately 25 protrusions per cm². This ply confers and gives thesheet thickness and a certain stiffness and good mechanical integrity.

The third ply 3 (or group of plies) forms the second external surface ofthe sheet and may advantageously have microembossing, that is to sayembossing comprising at least 30 protrusions per cm².

The third ply thus makes it possible to limit the two sided effect, thatis to say the visible trace of the embossing of the first ply on thethird ply, and above all avoids there being a sensation that isunpleasant to the touch.

Each ply may or may not have the same nature.

The expression “group of plies” means that each of said first, secondand/or third aforementioned plies may in fact consist of one or severalplies.

Furthermore, the plies 1, 2, 3 are glued together at dots and/or linesof glue 4. The nature of the glue and its dilution ratio are known perse and can be readily determined by the person skilled in the art aspart of his everyday know how.

Advantageously, the glue is deposited on one of the internal surfaces ofthe sheet as can be seen in FIG. 1.

FIG. 2 schematically shows an installation capable of producing sheetsaccording to one embodiment of the invention. The associated processwill be described in relation to this FIG. 2.

Thus, each of the plies 1, 2, 3 is embossed in a way known per sebetween an engraved steel cylinder 51, 61, 71 and a rubber cylinder 52,62, 72.

Each of the plies was wet laid beforehand using the technology wellknown to those skilled in the art of the CWP (conventional wet press)type.

By way of illustration, the first ply 1 is embossed in a nip 5 betweenthe first engraved cylinder 51 and the first smooth cylinder 52, theengraving of the first cylinder 51 being such that a somewhat decorativeembossing pattern (patterns) is produced on the first ply 1 (or group ofplies) which will constitute one of the external surfaces of the sheet.

The other external surface of the sheet consists of the third ply 3 (orgroup of plies) which is embossed in a nip 7 between the third engravedcylinder 71 and the third smooth cylinder 72. The engraving of the thirdengraved cylinder 71 is such that microembossing is for example formedon the third ply (or group of plies) 3. In the conventional way,microembossing is defined by a protrusion density greater than or equalto 30 per cm².

Furthermore, the second ply 2 (or group of plies) is inserted betweenthe first and third ply.

The second ply is embossed between the second embossing cylinder 61 andthe second smooth cylinder 62 at a second nip 6.

Preferentially, the embossing obtained is coarse microembossing asalready mentioned hereinabove.

Once embossed, the second ply 2 is directed towards an adhesive-coatingunit 8 known per se which essentially comprises a doctor cylinder 82 andan applicator cylinder 81.

The adhesive-coating unit 8 is preferentially positioned under the firstengraved cylinder 51 so that the applicator cylinder 81 collaborateswith the first engraved cylinder 51 at a nip 10.

More specifically, the applicator cylinder 81 bears against the engravedcylinder 51 via the second ply 2 and the first ply 1 superposed. Theplunger cylinder 82 transfers the adhesive from the glue reserve of theunit 8 to the applicator cylinder 81. The applicator cylinder 81 exertsa certain pressure on the engraved cylinder 51 at the distal surface ofthe protrusions of the first embossed ply 1. With sufficient pressure,the glue passes through the second ply 2 and the association of plies 1and 2 is thereby improved.

In an embodiment variant, the glue applicator cylinder 81 may have anengraved surface with a view to applying glue only to part of the topsof the engravings. By reducing the area coated with glue it is thuspossible to improve the flexibility of the end product.

The adhesive 4 used may be a standard glue of the PVA type or of the hotmelt type. By way of example, use was made of a glue marketed by thecompany Swift. This glue was diluted in water in optimum proportions toobtain appropriate transfer to the plies.

In another embodiment, appropriate means are used to spray a hot meltadhesive to each of the faces of the second ply. In this case, theadhesive needs to be applied before the ply 2 comes into contact withthe other two plies.

According to yet another embodiment, an aqueous adhesive is applied byspraying onto the central ply 2.

Having passed through the nip 10, that is to say once they have beenassociated with one another, the plies 1 and 2 are associated with thethird embossed ply 3.

The plies pass first of all (in the direction of the arrow 11) betweenthe first engraved cylinder 51 and the third engraved cylinder 71 andthen are associated at a nip 12 via spots of glue 4 by virtue of amarrying cylinder 9 which collaborates with the first engraved cylinder51.

This association is performed between the marrying cylinder 9, which isless hard than the first embossing cylinder 51, at a specific pressureof the order of 10 kg/mm².

It should be pointed out that, according to the invention embodimentjust described, the at least three plies (or group of plies) that formthe sheet have each undergone a different deformation during thetransformation or converting phase.

This characteristic introduces both body, gentleness and an harmoniouslook to each sheet.

Without departing from the scope of the invention, the plies may beassociated by knurling them between (hard) engraved knurling wheels andsmooth cylinders which are less hard. Deformation by knurling is of amechanical type but then affects only strips of greater or lesser widthon the plies.

In general, association by knurling is not as good as association byembossing and/or adhesive and the impression of relief defined by aknurling operation is not as visible as it is when the association isdone by embossing and/or adhesive.

By way of illustration, U.S. Pat. No. 3,377,224 discloses a conventionalknurling process.

In accordance with the invention, association of the plies may beperformed by a so-called “dry marking” technique for example accordingto the teachings of patent EP 864 014 B1: in this known method, theplies that are to be associated are passed in turn between asubstantially elastic receiving first cylinder and an engraved cylinderand then between this same cylinder and a substantially elastic fixedsecond cylinder; since the receiving first cylinder and the engravedcylinder are able to move and the elastic second cylinder is fixed, theassociation of plies obtained is such that the sheet exhibits a smoothface and a face of reduced thickness bearing impressed patterns.

The particular feature of the process according to the invention is theability to associate at least three plies or groups of plies. Morespecifically, the total number of plies may be N=n+m+p; n, m, p eachbeing an integer number, preferentially and nonlimitingly rangingbetween 1 and 3. Favorable results have been achieved with rollsproduced using sheets comprising between 3 and 12 plies.

According to the invention, the plies are advantageously associatedusing a single deposition of glue, when each of the plies is obtained bya wet lay technique of the CWP type.

Another advantageous feature of the invention lies in the fact that theabsorbent sheet can disintegrate, that is to say readily breaks up inwater, in spite of the high number of plies of which it is formed. Thedistintegratability (disintegration) within the meaning of the inventionis, in particular, that defined by French standard NF Q 34-020. Thisstandard applies to items for sanitary and domestic use, particularlybathroom tissue and relates to how readily such products disintegrate.

According to a notable feature of the invention, the absorbent sheetshave a coefficient K_(NOVE)=R_(SM)×R_(ST)×A×G×Exp (12×(E+E_(p)))×(1/Exp(SP))>75 000 where

-   -   R_(ST)=tensile strength of the sheet in the cross direction of        the sheet, in N/m    -   R_(SM)=tensile strength of the sheet in the machine direction of        the sheet, in N/m    -   G=basis weight of the sheet in kg/m²    -   E=thickness of the sheet in mm    -   Sp=flexibility of the sheet in N    -   A=absorption of the sheet in kg/m²    -   E_(P)=mean thickness of one ply of the sheet in mm, E_(P) being        greater than 0.115 mm.

All the sheets the characteristics of which meet this technicaldefinition simultaneously display thickness, softness and good strength.It is entirely unexpected for these characteristics to be foundsimultaneously in an absorbent sheet that disintegrates readily.

This is why the conceivable applications of the invention are numerousand varied: bathroom tissue, kitchen towel, cleaning product, makeupremover.

The “tensile strength” or, more specifically, the “resistance to breakby tensile” defined in the standard NF EN 12625-4 part 4 is the maximumtensile force withstood per unit width by a specimen of tissue paper ortissue product until it breaks, in a tensile test. The principle of thistest is that a specimen 50 mm wide and at least 150 mm long is stretchedto breaking point at a constant rate of elongation of 50 mm/min.

To do this, the specimens to be evaluated are placed in turn between twojaws of a tester (as defined precisely in the standard EN 12625-4): atleast 20 specimens are subjected to the test.

The jaws of the tester separate from one another at a constant rate ofabout 50 mm/min and all the significant values of the forces uponbreakage of the specimens are recorded.

Next, the mean of said breaking forces: F (expressed in N) is calculatedin order to determine the mean tensile strength Fm expressed in N/m suchthat:

${Fm} = \frac{F \times 10^{3}}{wi}$where wi=the initial width of the specimen (normally 50 mm).

As far as the flexibility Sp of the sheet is concerned, this beingexpressed in N, this is determined by a so-called “ring and rod” methodwhich consists in passing the sheet of surface area S through a hole ofa diameter slightly greater than the large diameter of a cone frustumthat the sheet will wrap around. The tensile force on the sheet beforeand during its passage through the hole is measured and plotted on agraph on which the movement of the test cage is plotted on the abscissaaxis (in mm) and the tensile force is plotted on the ordinate axis (inN). As the sheet passes through the hole, the gradient of the curvechanges and the magnitude of the force at the point of inflection isnoted. This value is correlated with representative of the flexibilitySp (in N) of said sheet because it is inversely proportional to saidforce.

The Sp flexibility of a test sample is measured using a test cage, cone(frustum) and rod as illustrated in FIG. 4 wherein test cage 100 has aheight of 100 mm. Square base plate 102 is 120 mm x 120 mm and has athickness of 3 mm with flaring hole 104 formed therethrough. Mouth 106of flaring hole 104 has a diameter of 40.5 mm while exit 108 has adiameter of 38.5 mm, the radius of curvature thereof being 33 mm so thatthe tangent to flaring hole 104 is horizontal at its opening andvertical at its exit. Cone (frustum) 110 is 25 mm in diameter at its topand 33 mm in diameter at its bottom with a height of 30 mm. Rod 112 hasa diameter of 6 mm. Standard test sample 114 is a square 100 mm x 100 mmwith hole 116 having a diameter of 8 mm formed at its geometric center.In operation, test sample 114 is centered on base plate 102, rod 112 ispassed upwardly through hole 116 in test sample 114 and secured to cone(frustum) 110 as shown in FIG. 5. As test cage is pulled upward by adynamometer at a rate of 400 mm/min, the force required to allow thesheet to pass through the hole 104 and be wrapped around the cone ismeasured and the rigidity value obtained as indicated above at the firstchange of slope, is representative of the flexibility.

E is the measured thickness of the sheet obtained in accordance withEuropean standard EN 12625-3 while Ep is the mean thickness of a ply ofsaid sheet. More specifically, Ep is the total thickness of the sheetdivided by the number of plies of which the sheet is formed; hereconsidered to be greater than 0.115 mm.

A is the absorption of the sheet, in kg per m², that is to say the massof liquid absorbed per unit area of the sheet. This parameter isdetermined on the basis of the conditions set out in the standard EN12625-8 in that the absorption is thus measured in grams of liquid pergram of paper and if the basis weight of the paper is also known (as aweight per unit area), then multiplying these values together leads toan absorption in weight per unit area, as given in the table whichfollows.

Furthermore, as far as its softness is concerned, the absorbent productaccording to the invention can be characterized by a coefficientK_(D)=K_(NOVE)×Exp (D)>100 000, D being a number ranging between −3 and+3 and obtained according to the following test:

The individual testing the product chooses an adjective to describe thesoftness on a verbal scale. The method consists in allocating marks onthis verbal scale according to the following look-up table, comparingeach of the test sheets against the same control 0:

Verbal scale/score Far less soft −3 Less soft −2 Probably less soft −1Not different 0 Probably softer +1 Softer +2 Much softer +3

The number of individuals who have chosen a given softness adjective isthen multiplied by the score corresponding to that adjective. Then thepoints obtained are summed and divided by the total number ofindividuals in order to obtain an average score. This average score D,which lies between −3 and +3, gives the result of the test for a givenproduct.

The table which follows gives values for various types of sheet obtainedaccording to the invention, and for various known products.

Basis Product Strength Strength Flexi- Absorption Thickness/ Numberweight thickness SM ST Soft- bility K_(n) K_(D) Product kg/m² ply mm ofplies kg/m² mm N/m N/m ness SP in N (NOVE) (softness) Invention 9 plies0.00120 0.123 9 0.148 1.11 1017 392 1.0 3.1 8965523 24370818 5 plies0.00112 0.186 5 0.097 0.93 396 242 1.5 1.5 1514893 6789278 (2TAD +1CWP + 2TAD) 7 plies 0.00091 0.134 7 0.114 0.94 704 234 1.0 1.9 10688572905455 6 plies 0.00092 0.150 6 0.110 0.90 580 260 1.0 1.9 7147131942791 5 plies 0.00096 0.154 5 0.090 0.77 439 365 1.5 1.5 200804 899942(1TAD + 3CWP + 1TAD) GP prior Moltonel 0.00045 0.207 3 0.051 0.62 350180 1.2 0.7 15306 50819 art Royal 0.00044 0.173 3 0.052 0.52 322 173 1.00.7 2579 7011 Lotus comfort 0.00035 0.160 2 0.041 0.32 235 97 2.2 0.2 82743 Lotus petite 0.00033 0.165 2 0.040 0.33 208 87 2.2 0.4 61 551 fleur

This table clearly reveals that the invention, particularly through thepredefined coefficients K_(NOVE) and K_(D), differs from the prior art.The invention makes it possible simultaneously to improve the strengthof the product, its thickness and its softness.

Furthermore, the sheets according to the invention are rectangular orsubstantially rectangular and have a tensile strength greater than about700 N/m in their machine direction and at the same time greater thanabout 300 N/m in their cross direction.

These values can be considered to be high by comparison with knownvalues: the 4-ply bathroom tissue marketed in Germany under the tradename “Servus” by Kimberly-Clark and which has a machine directionstrength of 605 N/m is, for example, known. This is one of the highestvalues for bathroom tissue currently on the market, as demonstrated inthe above table.

The tensile strength of this same paper in the cross direction lies ataround 208 N/m.

In terms of tensile strength of each sheet in the cross direction, theproduct marketed in Germany under the trade name “Hakle Ultra Care” byKimberly-Clark has a cross direction strength of about 283 N/m, whichvalue is considered to be very high. By contrast, this paper has amachine direction strength of about 593 N/m.

Thus, the present invention displays exceptional tensile strength bothmachine and cross directions, together with other characteristicsparticularly the characteristic relating to the coefficient K_(NOVE).

The characteristic relating to the tensile strength obtained accordingto the invention, which may or may not be combined with othercharacteristics, is favorable to allowing just one sheet to be used at atime, rather than several as has been the case hitherto.

Surprisingly and unexpectedly, the aforementioned strengths do not inany way diminish the softness of the product obtained, or itsflexibility. The above table shows the improvement that the inventor hasmade in this area.

The preferred thickness exceeds 0.75 mm and is preferentially less than1.5 mm while the basis weight is higher than about 70 g/m² andpreferably lower than 150 g/m².

These values, which are high by comparison with most known products,heighten the “use one” nature of how the product is consumed.

In order to encourage sheet by sheet use still further, in the case ofbathroom tissue, a size of 140 mm×110 mm is preferred. This dimension issomewhat larger than the usual size.

According to one embodiment of the invention, the absorbent sheets arejoined together by precuts and together form a roll.

Given the intrinsic characteristics of the sheets as explainedhereinabove in particular, the precut ratio may be chosen at about 0.15per mm. This ratio allows each sheet to be separated easily andreliably.

In the case of an application to bathroom tissue, since the inventionadvantageously makes it possible to use just one sheet at a time, rollscomprising a lower number of sheets, that is to say between 60 and 120rather than the customary 150, are formed; this reduction not in any waybeing penalizing to the user.

Furthermore, surprisingly and unexpectedly, the plies that form thesheets according to the invention do not separate from one another whilethe rolls are being formed. This seems in particular to be due to theirgood association and their good flexibility and elasticity.

According to another embodiment of the invention, the sheets may bestacked, then possibly wrapped to form packets.

Advantageously, according to the embodiment illustrated in FIG. 2 at themarrying device 9, the plies are associated with one another under aspecific pressure of about 10 kg/mm². The specific pressure is moreparticularly applied to points and/or lines of adhesive coatinguniformly distributed in the cross direction, along generatrices of theengraved first cylinder 51.

More specifically, the specific pressure needs to be constant over theentire developed surface area of the first cylinder 51 in order inparticular to reduce cylinder vibration and therefore wear.

Depending on the final characteristics and nature of the sheet obtained,the specific pressure may vary from 3 to 16 kg/mm².

By way of illustration, sheets comprising 9 plies have been manufacturedin accordance with the invention. Their basis weight is about 150 g/m²and their thickness is about 1 mm. The specific pressure at the marryingdevice was therefore about 11 kg/mm². The corresponding manufacturingprocess was carried out using an installation as depicted schematicallyin FIG. 2 and explained above. With respect to FIG. 1, each of the plies1, 2 and 3 here consisted of three plies joined together, in a way knownper se, upstream of each embossing unit (51, 52; 61, 62; 71, 72).Furthermore, each ply was wet laid beforehand using a CWP technique.

The multi-ply absorbent sheet according to the invention may compriseonly plies formed in this way, which are ultimately associated with oneanother in accordance with FIG. 2.

Without departing from the scope of the invention, the plies may be wetlaid using a TAD (through air drying) technique known per se and whichconsists, in general, in throwing a suspension of papermaking fibersonto a fabric in order to form a web, in transferring this web onto thesurface of a porous drying cylinder, in forcing jets of air from saidcylinder through the web and then in transferring the dry web thusformed onto a so-called creping cylinder. One or more plies are formedin this way.

Wet laid plies according to the teachings of application EP 1 353 010may also form part of a sheet according to the invention.

The plies may be associated using an installation according to FIG. 2which may or may not comprise an adhesive-coating unit.

The invention is also aimed at sheets comprising both plies obtainedaccording to a conventional (CWP) technique and plies produced using theTAD technique.

The plies of the TAD type may constitute the two outer faces of suchsheets. A very soft feel is therefore obtained. The inside plies may beproduced using a CWP or some other technique.

Without departing from the scope of the invention, the outer plies maybe wet laid using a conventional technique (CWP) and form the two outerfaces of the sheet. In this configuration, the inner plies may beproduced using a TAD type of technique.

Of course, a combination of plies aimed at producing a sheet that hasone face formed of a conventional ply and one face formed using a TADtype technique may also be produced without departing from the scope ofthe invention.

One of the possible combinations will be chosen according to theparticular circumstance, that is to say according to the feel, theabsorption, the flexibility mainly desired, or alternatively accordingto constraints of an economic nature.

Furthermore, it is perfectly conceivable to produce a sheet according tothe invention without using adhesive to associate the plies, whateverthe nature of each of the plies (or group of plies) of which said sheetis made.

1. A substantially rectangular multi-ply disintegratable absorbent sheetbased on tissue comprising at least five plies including two exteriorplies and at least three interior plies, each of at least three of theplies in said sheet being in at least one group of plies, each of saidat least three plies being joined to another ply by mechanicaldeformation, characterized in that: the mean thickness of one ply of thesheet is greater than 0.115 mm; the basis weight of said sheet isgreater than 0.070 kg/m2; and the total product thickness exceeds 0.75mm.
 2. The substantially rectangular multi-ply disintegratable absorbentsheet of claim 1 wherein each of the two exterior plies of said sheetare TAD plies.
 3. The substantially rectangular multi-plydisintegratable absorbent sheet of claim 2 wherein each said exteriorply is in a group of plies and is joined to another TAD ply bymechanical deformation.
 4. The substantially rectangular multi-plydisintegratable absorbent sheet of claim 2 wherein the three interiorplies are CWP plies joined together by mechanical deformation.
 5. Thesubstantially rectangular multi-ply disintegratable absorbent sheet ofclaim 4 wherein the three interior plies are joined together bymacro-embossing.
 6. The substantially rectangular multi-plydisintegratable absorbent sheet of claim 2 wherein each said exteriorply is in a group of plies and is joined to another TAD ply bymechanical deformation and wherein the interior plies are CWP pliesjoined together by mechanical deformation.
 7. The substantiallyrectangular multi-ply disintegratable absorbent sheet of claim 2 whereinthe groups of plies are joined to each other by adhesive.
 8. Thesubstantially rectangular multi-ply disintegratable absorbent sheet ofclaim 2 wherein at least one of said exterior plies bears amicro-embossing pattern comprising at least 30 protrusions per cm². 9.The substantially rectangular multi-ply disintegratable absorbent sheetof claim 2 wherein each group of plies is embossed independently of theother groups of plies and each bears a distinct embossing pattern. 10.The substantially rectangular multi-ply disintegratable absorbent sheetof claim 2 wherein the groups of plies are joined to each other byknurling.
 11. The substantially rectangular multi-ply disintegratableabsorbent sheet of claim 1 wherein each of the plies of said sheet areCWP plies and each CWP ply is joined to another CWP ply by mechanicaldeformation.
 12. The substantially rectangular multi-ply disintegratableabsorbent sheet of claim 11 wherein the groups of plies are joined toeach other by knurling.
 13. The substantially rectangular multi-plydisintegratable absorbent sheet of claim 11 wherein each CWP ply isjoined to two other CWP plies by mechanical deformation.